The US Supreme Court has said Donald Trump and other former presidents are partially immune from criminal prosecution, in a major legal victory for the Republican White House candidate.

The 6-3 ruling did not outright dismiss an indictment that charges Trump with plotting to overturn the 2020 election, but it did strip away key elements of the case against him.

The justices found that a president has immunity for “official acts”, but is not immune for “unofficial acts”, and referred the matter back to a trial judge.

The three liberal justices dissented strongly, expressing “fear for our democracy”.

“The President is now a king above the law,” wrote Justice Sonia Sotomayor.

The decision makes it less likely that the Republican candidate will stand trial in the case before he challenges Democratic President Joe Biden in November’s White House election.

It is the first time since the nation’s founding that the Supreme Court has declared former presidents can be shielded from criminal charges.

Trump is the first president ever to be criminally prosecuted, as Chief Justice John Roberts noted while delivering Monday’s opinion.

“Big win for our constitution and democracy,” wrote Trump in an all-capital letters post on his social media platform Truth Social.

In a fiery phone call with the media, deputy Biden campaign manager Quentin Fulks could be heard banging his fist on the table as he spoke.

“Immune, immune, immune. They just handed Donald Trump keys to a dictatorship,” Mr Fulks said, pointing out that three of the justices had been appointed by Trump.

Special Counsel Jack Smith, who filed the indictment, declined to comment.

The majority opinion by the highest court in the land tossed out a lower court opinion that had rejected Trump’s claim of absolute immunity.

The justices found a president does enjoy absolute immunity for official conduct, but can still be prosecuted for private acts.

Justice Roberts wrote that a president’s discussions with the Department of Justice are official acts of the presidency, and he or she is therefore “absolutely immune” from prosecution for such interactions.

The indictment alleges Trump pressured the law-enforcement agency to investigate claims – which were found to be unsubstantiated – that widespread voter fraud had affected the election result.

Justice Roberts wrote that a president’s discussions with his vice-president are also official conduct, and Trump is therefore “at least presumptively immune” from allegations that he tried to pressure Mike Pence not to certify Mr Biden’s victory in the 2020 election.

The indictment accuses Trump of inciting the US Capitol riot, citing his tweets and remarks he made outside the White House that day.

But the Supreme Court ruled on Monday that Trump’s speech and social media activity on 6 January 2021 were all official acts.

In another blow to the case, the justices ruled that Trump’s private records – and those of his advisors – “may not be admitted as evidence at trial”.

The opinion raised questions, too, about whether allegations that Trump pressured state officials to change their electoral votes in order to overturn his election defeat constituted unofficial acts, but ultimately left it to the lower court to decide.

“The parties and the District Court must ensure that sufficient allegations support the indictment’s charges without such conduct,” said the opinion, raising doubts about the potential viability of the case once the official acts are stripped away.

In her dissent, Justice Sotomayor argued that the ruling would protect a president if he or she ordered US special forces to assassinate a political rival, organised a military coup to hold on to power, or took bribes in exchange for conferring a pardon.

Justice Jackson wrote in a separate dissent that the conservative majority’s ruling “breaks new and dangerous ground” and would “let down the guardrails of the law”.

But Justice Roberts wrote that the “tone of chilling doom” from the dissenters was “wholly disproportionate”.

His opinion said that immunity extends to the “outer perimeter” of the president’s official responsibilities, setting a higher bar for prosecution.

This ruling is “among the worst-case scenarios” for the special counsel, says Aziz Huq, a constitutional law expert at the University of Chicago.

“I think it will be important to see if [Jack] Smith can narrow the indictment by eliminating those facts that the Court has ranked as ‘official’,” he told the BBC.

“This is a major victory for Donald Trump,” legal expert Mitchell Epner told the BBC.

He said the trial judge will now have to decide which charges can move forward, and Trump will be able again to appeal against her ruling all the way to the Supreme Court.

Additional reporting by Kayla Epstein